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	<h1 id="firstHeading" class="firstHeading" lang="en">Straight line control points</h1>
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The panorama tools optimizer understands various different schemes for
aligning photos using <a href="Control_points.html" title="Control points">control points</a>:
Normal points are <b>t0</b> points, <a href="Horizontal_control_points.html" title="Horizontal control points">horizontal control points</a> are <b>t1</b> points,
<a href="Vertical_control_points.html" title="Vertical control points">vertical control points</a> are <b>t2</b> points, and straight line control points are <b>t3</b>, <b>t4</b>, etc... points.
</p><p><b>Straight line control points</b> were added later and have a similar effect as horizontal
and vertical points, except that the straight line can be at any angle.  They have two main uses:
Lining up linear features that don't have identifiable detail, such as the edges of a room or
overhead cables; and calibrating <a href="Lens_distortion.html" title="Lens distortion">lens distortion</a> using a single
<a href="Rectilinear_Projection.html" title="Rectilinear Projection">rectilinear</a> photograph of a grid or building.
</p><p>Although two points are sufficient to define a horizontal or vertical line, more are
required for these lines at arbitrary angles.  Since the script file format only supports 'pairs' of
points, a straight line is generally defined with four, six or eight etc... control points.
</p><p>Some of the GUI front-ends<a class="external" href="https://wiki.panotools.org/GUI_front-ends">[*]</a> support placing these <b>straight line control points</b>, see this
tutorial on <a rel="nofollow" class="external text" href="http://www.erik-krause.de/verzeichnung/distort_en.htm">correcting lens distortion with ptgui</a>.
</p><p>Otherwise if you edit a <a href="PTOptimizer.html" title="PTOptimizer">PTOptimizer</a> script and make more than one pair of existing points
<b>t3</b>, then the optimiser will try to line them up as a straight
line in the output image.  You can keep going, with <b>t4</b>, <b>t5</b> etc... for each group of points
that you want in different 'straight lines'.
</p>

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